In cellular telephone systems, the served area is divided into cells, each of which may be further divided into sectors. Each cell is served by a single base station and all of the base stations are connected to a message switching center ("MSC") via a base station controller ("BSC") and hardware links. A plurality of mobile units are connected to the MSC by establishing radio links with one or more nearby base stations.
In earlier cellular telephone technology, such as time division multiple access ("TDMA"), as a mobile unit traveled from one cell to another, the radio link between the mobile unit and the base station serving the first cell had to be broken then replaced by a radio link between the mobile unit and the base station serving the second cell. In contrast, in a code division multiple access ("CDMA") cellular telephone system, because the same frequency band is used for all cells and sectors, the first link need not be broken before connecting with the second link. The CDMA waveform properties that provide processing gain are also used to discriminate between signals that occupy the same frequency band. A mobile unit thus need not switch frequencies when a call is transferred from one cell or sector to another. Additional details regarding the specifics of the CDMA cellular telephone environment are described in TIA/EIA/IS-95-A, Mobile Station-Base Station compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System (hereinafter "CDMA Standard"), which is hereby incorporated by reference in its entirety.
In the context of a cellular telephone system, "handoff" is the process of handing over a call from one sector to another when a mobile unit detects that acceptable communication with the other sector is possible. This occurs mainly when the mobile unit nears a sector boundary or the current communication link is weakened by radio frequency ("RF") shadowing and another potential communication path from another sector is enhanced. In general, handoff consists of three phases. During the first phase, referred to as "handoff initiation," the handoff process is triggered. During the second phase, referred to as "target selection," a determination is made which sectors are candidates for receiving the handoff. During the third and final phase, referred to as "handoff completion," the mobile unit is transferred from the old sector to the new sector.
The term "soft handoff" is commonly used to refer to a handoff in which the mobile unit commences communication with a new base station without interrupting communications with the old base station, i.e., the call is maintained on both base stations. If there are three cells involved in the handoff, the call will be maintained by all three base stations. A "softer handoff" refers to a handoff in which the call is maintained on one base station for different sectors of the same cell. A hybrid form of the previously discussed types of handoff, referred to as a "soft/softer handoff", results if there are two sectors from one cell and another sector from another cell involved in the handoff, in which case two base stations are involved. The terms "handoff" and "soft handoff" will hereinafter be used interchangeably to refer to all of the foregoing types of handoff.
Each sector of the CDMA system continuously outputs its own unique pilot signal. A mobile unit can distinguish between the sectors by the pilot signals emitted thereby and can also measure the strength of the pilot signal by measuring the carrier-to-interference ratio ("C/I") thereof. The strength of the pilot will indicate whether or not the sector with which it is associated can be used by the mobile unit to establish communication.
To successfully complete a soft handoff, the mobile unit must detect suitable sectors for handoff and keep track of sectors with which it is currently in communication. In other words, successful handoff involves determining the point at which a sector can communicate acceptably with the mobile unit, as well as the point at which a sector can no longer be used by the mobile unit. This is accomplished by the mobile unit's continuously measuring the strengths of the pilot signals of the sectors of interest. Since the mobile unit can generally only search sectors one at a time, the searching is governed by a set of rules.
In particular, the mobile unit searches for pilot signals on the current CDMA frequency assignment to detect the presence of pilot signals and then measures their strengths. When the mobile unit detects a pilot of sufficient strength, meaning that it has a C/I above a certain minimum "ADD threshold", that is not associated with any of the forward traffic signals currently assigned to it, it sends a "pilot strength measurement message" ("PSMM") to the sector(s) with which it is currently in communication. The BSC will then assign a forward traffic channel associated with that pilot signal through the base station to the mobile unit and direct the mobile unit, via a "handoff direction message" ("HDM"), to perform a handoff. The mobile unit acknowledges the HDM with a "handoff completion message" ("HCM") and begins communicating with the new sector while maintaining communication with the old sector(s). In the event that the C/I of one of the sectors with which the mobile unit is communicating drops below a "DROP threshold" and stays below that threshold for a time of "T-TDROP" seconds, the mobile unit sends a PSMM to the sector(s) with which it is currently communicating and the BSC will direct the mobile unit, via an HDM, to drop the weak sector. Receipt of the HDM is acknowledged by the mobile unit with an HCM.
The pilot signal search parameter rules are expressed in terms of various "sets" or "lists" of pilots, and sectors represented thereby, which include an "active set," a "candidate set," a "neighbor set," and a "remaining set." The active set comprises the pilots associated with sectors currently in communication with the mobile station. The candidate set comprises pilots that are not currently in the active set, but that have been received by the mobile unit at a strength sufficient to indicate successful communication. The neighbor set comprises the pilots that could be received with sufficient strength to enable successful communication; that is, pilots whose C/I can exceed the ADD threshold. Active and candidate set pilots will most likely have come from the neighbor set and pilots that are dropped from the active and candidate sets are placed in the neighbor set. The remaining set comprises all possible pilots in the system on the current CDMA frequency assignment, excluding those pilots in the active, candidate and neighbor sets. The active and candidate sets can include a maximum of six (6) and five (5) pilots, respectively, while the neighbor set maximum is typically twenty (20) pilots.
Throughout a call in progress, the mobile unit searches and measures the strength of the pilots in the various sets; however, the mobile unit searches the active set most often, as the maintenance of the call undoubtedly depends most heavily on the sectors in the active set. The candidate set is also searched fairly often, although not as often as the active set. The neighbor set is searched even less often than the candidate set. For obvious reasons, the remaining set is rarely searched as compared to the other sets, and, since the remaining set is far larger than the other sets, the time between remaining set pilot searches is much longer than the time between neighbor set pilot searches.
Each time the mobile unit is ordered to add and drop pilots, the neighbor set will likely need to be updated, a task which is performed by the BSC. The updated neighbor set is sent as a "neighbor list update message" ("NLUM") as regular traffic on the communication link between the sector(s) with which the mobile unit is currently communicating and the mobile unit itself.
In a CDMA cellular telephone system, efficiently performing a soft handoff involves optimizing the choice of pilots and sectors to be included in the neighbor set. The inability to optimize the neighbor set is detrimental to the overall performance of the system, the direct impact of which is an increase in the number of dropped calls and a decrease in call quality. Accordingly, optimization is crucial in at least two regards. First, the erroneous inclusion of a pilot in the neighbor set will undoubtedly cause the mobile unit to waste valuable time searching for this pilot. Moreover, since the neighbor set is usually constrained in size to minimize the overall time searching for favorable neighbors to which to handoff a call, a valuable slot is effectively wasted. Still further, the erroneous exclusion of the sector from the neighbor set, i.e., the inclusion of it in the remaining set, increases the likelihood of a call being dropped, as remaining set members are rarely scrutinized. Accordingly, a potential neighbor set member that is erroneously included in the remaining set will most likely be ignored until it is too late.
At this point, an explanation of why the call quality and probability of maintaining a call is detrimentally affected by a less than optimal neighbor set is deemed to be appropriate. In particular, if a sector is favorable for handoff, as indicated by a strong pilot signal, but the pilot is not detected during a search because it was erroneously excluded from the neighbor set, one of several situations will result. First, as the mobile unit continues to move, the pilots in the active set will lose strength due to the contribution of the undetected favorable pilot as interference. On the other hand, if the undetected pilot is detected and added to the active set, although the other pilots will continue to lose strength, call quality will be maintained or improved, as the favorable pilot is now involved in the communication. Second, as the mobile unit moves and the undetected pilot increases in strength, the sectors included in the active set might be too weak to sustain adequate communication, resulting in the call being dropped. Finally, if the undetected pilot is finally detected as the mobile unit is moving, a PSMM will be sent; however, if the communication link has been degraded beyond a certain point, the HDM sent by the active sector(s) instructing the mobile unit to add the new sector to its active set will not be received and the call will be dropped.
For each individual sector in the system, a neighbor set is determined and its members prioritized or ranked in order of importance based primarily on the position and direction of the antenna of the potential neighbor set member, the distance of the potential neighbor set member from the sector of interest and the effects of shadowing in the path therebetween. For example, referring to FIGS. 1A and 1B, in which each sector, such as a sector S, is represented by a hexagon, and wherein each cell, such as a cell C (FIG. 1A), comprises three sectors, one or more antennas for each sector are located at or about a point P (FIG. 1A) in the center of the cell C and are aimed at the opposite corner of the sector. For example, with respect to the sector S, its antenna or antennas are aimed in a direction represented by an arrow A such that the pilot of the sector S is strongest in the direction indicated by the arrow A.
As previously indicated, to determine which sectors are "important" neighbors, proximity information, antenna position and direction and shadowing effects must be considered. A sector displaced from the sector of interest by two other sectors may be important if its antenna is pointing in the direction of the sector of interest and there is minimal shadowing in the path therebetween. FIG. 1B illustrates the neighbor set of the sector S, in which the relative priority or ranking of each neighbor set member is represented by a number therein. For example, a sector labeled "1" has a priority of 1 in the neighbor set of the sector S. It will be easily recognized that the sectors bordering the sector S to the left and right thereof have a priority of 1, due to their proximity to the sector S and not unfavorable antenna directivity. The two sectors immediately below the sector S have a priority of 2, due also to their proximity and not unfavorable antenna directivity. In contrast, the two sectors immediately above the sector S have a priority of 7. This is due to the fact that, although the sectors are proximate to the sector S, their antennas point directly away from the sector S. Conversely, the sector above these two sectors, although not as close to the sector S, has its antenna pointed directly toward the sector S, and therefore has a priority of 3.
As illustrated above, each individual sector has its own neighbor set which may be empirically identified and prioritized through observation and measurement of pilot signal strength, antenna position and direction, the distance between the sectors, and the effect of shadowing in the path therebetween. Determining the neighbor set to be used when the mobile unit is in handoff between two or more sectors poses a more difficult problem. The apparent solution would be simply to combine the individual neighbor sets of the sectors involved in the handoff; i.e., the sectors in the active set. However, because the maximum size of the neighbor set sent by the BSC is limited to twenty (20), and because it is possible that one or more sectors in the active set might lie a considerable distance from the mobile unit, making its neighbors effectively useless, this "solution" is clearly deficient.
Accordingly, what is needed is a method of optimizing the neighbor set when a mobile unit is in handoff.